The use of fillers in both thermoplastic and thermoset polymers has been common. The practice of filling polymers is motivated both by cost reduction and by the need to obtain altered or enhanced properties (e.g. changes in thermal expansion coefficient, corona resistance, etc). Most conventional filler materials have dimensions that are larger than 1 μm. Certain materials (e.g inorganic oxides) are now available in nanometric dimensions.
Nanostructured dielectric materials have demonstrated advantages over micron-filled polymer dielectrics. For example, an increase in dielectric strength and a reduction in space charge have been documented for the case of nano-TiO2 filled epoxy over micron size TiO2 filled epoxy composites and titania filled polyethylene composites. Improvements in dielectric properties observed for nano-filled polymers could be due to several factors: (i) the large surface area of nanoparticles which creates a large ‘interaction zone’ or region of altered polymer behavior, (ii) changes in the polymer morphology due to the surfaces of particles, (iii) a reduction in the internal field concentration caused by the decrease in size of the particles, and (iv) changes in the space charge distribution and/or a scattering mechanism.
It is well known that polymer properties are altered near a surface. The high surface area of nanoparticles, therefore, leads to a large volume fraction of polymer with properties different from the bulk (the interaction zone). Depending upon the strength of the interaction between polymer and particle, this region can have either a higher or lower mobility than the bulk material. It has also been postulated that the free volume in such interaction zones differs from the bulk. Because these interaction zones are likely to overlap at relatively low volume fractions in nanocomposites, a small amount of nanofiller has been shown to result in both increases and decreases in glass transition temperature.